An essential part of the civilian GPS modernization effort has been to provide a second civilian signal on the L2 GPS carrier frequency, in addition to the course acquisition (C/A) signal currently on the L1 GPS carrier frequency. Initially, the modernization program planned to implement the GPS C/A signal on the GPS L2 carrier. Subsequently, alternate recommendations have been made to replace the C/A code on the L2 carrier with civilian codes having better correlation properties than the C/A code. Codes that have been considered for use on the L2 frequency include one of the L5 codes or the time division multiplexed (TDM) combination of two new codes with both codes chipped at the C/A chipping rate. The TDM codes include a civil-moderate (CM) ranging code and a civil-long (CL) ranging code that are significantly longer than the C/A code to improve the correlation and spectral properties as compared to C/A code. The new time division multiplexing (TDM) L2CS code has been specified to be implemented on the modified GPS satellites. The L2CS code improves the performance of both the high-volume single frequency and high-accuracy dual-frequency GPS receivers. The combination of the TDM CM and CL codes is used for code division multiple access (CDMA). The CM and CL spreading codes are multiplexed on a chip-by-chip basis. The CM code and CL code are combined as a composite spreading code that provides improved tracking performance superior to that of the C/A code.
The new L2 civilian signal L2CS is planned for use on the L2 GPS frequency to provide civilian users with improved ionospheric correction. The L2CS signal is a TDM combination of the moderate length CM code of 20.0 millisecond duration with data, and the long length CL code of 1.5 second duration without data. The composite code is much longer than the one millisecond C/A code for improved correlation properties resulting in superior cochannel interference rejection and less self noise. A dataless channel can provides 6.0 dB improvement in carrier tracking at low C/No, where C/No is the carrier power to noise spectral density ratio. In cell phones and personal data assistants, the L2CS signal is deemed essential for high-precision dual frequency GPS applications.
The TDM codes provide near continuous line spectra as compared to the C/A code, which concentrates power at one kilohertz spaced spectral lines. In order to obtain the ideal processing gain advantages of a spreading code, the code should exhibit near-continuous line spectra. When an interferer falls on one of the C/A code spectral lines, the ideal processing gain of C/A code is reduced. The effective C/No in the presence of an interferer having power I, is given by a C/No equation.
            (              C                  N          0                    )        eff    =            C                        N          0                +                  I          0                      =          C                        N          0                +                              (                          I              C                        )                    ⁢                      (                                          CT                i                            PG                        )                              
In the C/No equation, the term Ti is the coherent integration period, and the term PG is the processing gain in an ideal spreading process of a continuous line spectrum and is given by the product of the code rate, such as the 1.023 MCPS C/A code rate. The integration period is limited to 20 ms for the C/A code with data. Using the C/A code, the integration is continuous over time with the accumulated correlations dumped at a C/A dumping rate. The C/A code effectively loses processing gain when an interference source coincides with a C/A code spectral line while the L2 CL code is unaffected. The effective C/No gain reduction observed for the C/A code corresponds to a loss of processing gain as indicated by the C/No equation.
The L2CS TDM code receiver implementation uses a three-level reference code having +1, 0, and −1 levels with expected benefits of independently tracking either of the L2 CM or CL codes. In a conventional CDMA correlator, the received signal is multiplied by a two-level local reference code having +1 and −1 levels. The results are accumulated by integration over an integration and dump period and used to despread the CDMA signal for use in code acquisition, code tracking, carrier tracking and data demodulation circuits. During signal acquisition, prior to code tracking, the code phase of the local reference signal is shifted and the magnitude of the inphase and quadrature correlators is monitored until the correct code phase is detected. In a receiver, code tracking compares a code phase shifted replica of the code to the code received and continually adjusts the code phase so that the replica and received codes become synchronized. The correlation of early and late reference codes provides an input to a control loop for adjusting the local code phase until synchronized despreading of the received signal is achieved. Correlator implementations used in most low-cost high volume commercial GPS receivers typically employ two-level reference codes that are multiplied by the incoming signal using a multiplier, such as 2-bit by 1-bit multiplier. Such a multiplier can be simply implemented by modulating the sign bit of the incoming 2-bit I and 2-bit Q samples according to one of the two possible values of the reference code. This conventional two-level correlation process is well known.
In the L2CS codes implementation, the CM and CL codes are TDM into a composite code of alternating code chips. Because the code chips are interleaved, a means is necessary to correlate with the desired code, such as the TDM CM or CL code. The conventional method is to correlate the received signal with a reference code. The conventional two-level correlator technique for tracking two-level spreading codes using the conventional two-level replica codes, having values of +1 or −1, is replaced by a three-level correlator technique using a three-level reference code having values of +1, 0, −1, where the 0 value is placed on every other chip and used to mask out one code while tracking the other code. The segmented interleaved composite code appears as M1, L1, M2, L2, M3, L3, M4, L4, and so on, generated from respective CM and CL codes, where M1 is the first chip of the CM code and M2 is the second chip of the CM code, and so on. Similarly, L1 is the first chip of the CL code, L2 is the second chip of the CL code, and so on. In the receiver, during acquisition of the CM code, the replica CM code is generated using three levels, where the +1 and −1 levels define the replica CM code while the zero level masks out the alternating CL code chips. The three-level CM code replica is used in a three-level CM correlator. The three-level replica CM code is generated as M1, 0, M2, 0, M3, 0, M4, 0, and so on. After acquisition and tracking of the CM code, the replica CL code is generated using three levels, where the +1 and −1 values define the replica CL code and the zero levels masks out alternating CM code chips. The three-level CL code replica is used in a three-level CL correlator. The three-level replica CL code is generated as 0, C1, 0, C2, 0 C3, 0, C4, and so on. In effect, the three-level correlation process is used to ignore, that is, used to mask, the untracked code by multiplying by the zero value every other chip. Unfortunately, this three-level correlator technique increases the complexity and the required number of operations per second per correlator, used to acquire and track the TDM codes, relative to the complexity of existing C/A code correlator implementations used in many commercial C/A code GPS receivers. The increase in complexity is due to representing the spreading code by more than one bit to employ the three-level correlator approach, thereby preventing the simple sign-bit implementation of the multiplier used in conventional correlation processes. This increased complexity is especially detrimental for parallel-correlator GPS signal acquisition implementations that perform many correlations in parallel. Also, during code and carrier tracking and data detection operations, staggered correlations are processed in parallel using early, prompt, and late correlation processes. All of these correlation processes need three-level correlation implementations for mass consumer applications of GPS using the L2CS spreading code. These correlation processes can be used for GPS signal processing, such as in cell phones for emergency 911 applications. Parallel correlator technology is used in such receivers to enable reasonable signal acquisition times at low signal levels encountered when a cell phone is used in a building. Such applications are especially sensitive to power consumption increases that would result from increased complexity of the correlation process. Parallel correlator techniques are even more necessary to efficiently acquire the TDM signals because the code periods are many times longer than that of the C/A code. Given that power consumption is directly related to computational complexity, the complex circuits consume more power. Hence, it is desirable to use two-level correlation processes in GPS receivers. Thus, a more conventional two-level correlator implementation is highly desirable for use in reception of the L2CS TDM codes and represents the smallest change to existing commercial GPS receiver designs.
The CM code and CL code are TDM on a chip-by-chip basis. These codes were designed to service the needs of future low-cost high-precision dual frequency GPS users. In particular, the CM code is 10230 chips long and is transmitted at a 0.5115 MCPS rate with a period of 20 ms. The data, spread by the CM code, is 25 bits per second and is coded by a convolutional coder having a symbol period of 20 ms. For the TDM signals, the symbol period is 20 ms with the data being transmitted at 25 BPS. The CL code, on the other hand, is 767250 chips long and is transmitted at a 0.5115 MCPS rate. The entire code has a period of 1.5 seconds and is designed for high precision tracking using a phase locked loop. Theoretically, this multiplexing approach should provide an even energy split that is equivalent to a 3.0 dB loss per code, relative to no multiplexing between the CM and CL codes with no other measurable degradation in performance for the constituent codes. Tracking of the individual constituent codes is achieved using the three-level reference code with zeros in alternating chips of the code.
While the chip rate of the CM and CL codes is 0.511 MCPS, the 1.023 MCPS chip rate of the TDM combination is the same as the C/A code. However, the CM and CL codes are essentially transmitted at a 50% duty cycle with the CL code and the CM code respectively, each on half of the time, making the code chips appear to be chipped at the same rate as C/A code. The zeros in the three-level reference codes are used to enable the correlation and tracking of the CM code while ignoring CL correlations when the CL code is present, and for correlation of the CL code while ignoring CM correlations, when the CM code is present. The code properties of the composite CM and CL TDM code has been shown to be superior to that of C/A code. While this multiplexing approach with three-level reference correlation offers a means to acquire and track the CM codes, the receiver implementation is less efficient than that of a conventional C/A code receiver. This leads to excessive power consumption and increased costs because the implementation of a correlator using the three-level reference requires changing the multiplication operation used in existing C/A code receivers from 1.5 bit by 1 bit to 1.5 bit by 1.5 bit, which increases the receiver gate count. As a result, the increased power consumption is unsuitable for cell phones and small personal digital assistants. The existing chip-interleaved composite code disadvantageously uses a more complex three-level correlator in GPS receivers than a two-level correlator used in existing GPS receivers. These and other disadvantages are solved or reduced using the invention.